1. Field of the Invention
This invention relates to a system for reducing heat output in a specific segment of an internal wire impedance system for heating a pipeline.
2. Description of the Prior Art
Pipelines often require the fluid flowing in them to have lower viscosities than they would have at the ambient temperature of the pipe. In order to reduce the viscosity of the fluid, heat is generally transferred into the fluid. A way to achieve this is through steam tracing: a system which uses steam flowing in a separate conduit adjacent to the one transporting the fluid. Another system is one using alternating electrical current and the effects of a magnetic field produced by the current to increase the temperature of the fluid in the flow pipe. This second method has in the past been called "skin effect heating," or more correctly, "internal wire impedance heating."
Industry has used the skin effect or internal wire impedance heating which, under current practice, uses a ferromagnetic pipe attached substantially parallel and either interior of or exterior to a fluid-flow pipe. The ferromagnetic pipe has longitudinally extending through it an electrically insulated metallic wire that is electrically connected to the ferromagnetic pipe at a point remote from the point of entry of the insulated wire so that both the wire and pipe may be connected in series with each other and an alternating current (AC) source of power. Thus, the electric current flows through the insulated wire and returns through the wall of the ferromagnetic pipe. Due to the skin effect, most of the current flows near the inside wall of the pipe with essentially no current flowing at the outside wall.
Heat is generated in the wall of the ferromagnetic pipe by: magnetic hysteresis resulting from a type of internal friction as the magnetic domains within the pipe wall are reversed by reversals of polarity of the applied alternating current. This induces eddy currents to circulate throughout the pipe wall yielding an I.sup.2 R heating effect due both to such polarity reversals and return current flow through the pipe wall. Additional heat is also generated in the insulated wire according to Joule's Law, i.e., the I.sup.2 R effect of the current flowing in it.
A point worth mentioning here is the reason for using a pipe having the property called "ferromagnetism". It simply is that this property greatly increases the magnetic field in the pipe wall due to flow of alternating current through the conductor which results in significant heating by hysteresis and eddy currents. Examples of ferromagnetic elements are iron, nickel and cobalt. Additionally, some alloys may have components which by themselves are not ferromagnetic, but when combined together as an alloy show this property, e.g., MnBi.
The present invention includes several embodiments which reduce the heat output for a given segment without affecting the heat output of the adjacent pipe. The utilization of the present invention results in both an economical and efficient use of electrical power, such as where a heat reduction segment connects two or more noncontiguous fluid-flow pipes that are heated by a single heat-generating pipe. For example, a heated pipeline in a refinery may have a termination point a short distance away from a second heated pipeline which continues on to another place in the refinery. When a common internal wire impedance system is used for heating each of them, a heat-reduction section is desirable in the space between the two lines since there is no need to heat that space. It is also usable whenever less heat is required in a segment of a continuous fluid-flow pipe, such as a segment where the heat loss is less due to reduced size in a segment of the pipe, better thermal insulation, or a supplementary source of heat.